Bipolar membranes exhibit many of the electrical characteristics of a pn-junction. The bipolar membrane has a basic structure comprising two regions consisting of an ion exchange resin. Both regions are of an ionizable species, one region being of a kind which ionizes to yield mobile negative ions and immobile positive ions, while the other region is of the type which ionizes to yield positive mobile ions and negative immobile ions. Diffusion of mobile ions across the junction between the two regions produces a space charge region which is substantially free of mobile ions. The bipolar membrane in terms of its electrical characteristics has been analyzed in the prior art literature. See, for example, the article "Voltage Current Characteristics of Bipolar and Three-Layer Fixed Charge Membranes" by R. Simons, Biochimica Et Biophysics, Acta, 282 (1972) pages 72-79; "Space Charged Regions, Fixed Charge Membranes and the Indicated Property of Capacitance" by A. Mauro, Biophysical Journal 2: 179 (1962). Such bipolar membranes have been constructed by pressing together two strong acid and strong base membranes into a sandwich. A novel method of preparing a sandwich-type bipolar membrane is reported in Chemical Abstracts, Vol. 81, 176899 m (1974), where a conventional ion-selective membrane, of either the cationic or anionic type, is placed in an electrolytic cell with a finely ground ion exchange resin of the opposite selectivity, and a d.c. current applied across the cell to firmly coat the membrane with resin. More recently, a monolithic bipolar membrane has been produced in a single sheet of polyethylene which is hydrolyzed on one side by an NaOH solution in water and is aminated on the other side by a diamine or a polyamine. Such a bipolar membrane is described in detail in the article "Bipolar Membranes Made of a Single Polyolephine Sheet" by F. de Korosy and E. Zeigerson, Israel Journal of Chemistry, Vol. 9, 1971, pp. 483-497. With a cellulosic, non-ion exchange membrane and with a sodium styrene sulfonate ion exchange membrane, the selectivity for thiourea and urea has been changed by physically stretching the membrane, see M. E. Bogdanov and A. A. Efendiev, Ajob. Khim, Zh. 4:103, (1975).